TeamSwift

Yes Guys I'm currently building a mk2 with 12:1 75mm pistons. My car have a standard head on now have 198psi cyl 1 200 psi cyl 2 206 psi cyl 3 and 201 cyl 4 what will these 12:1 put me at?

I believe compression readings should increase in proportion with the increase in compression ratio. I don't know the stock compression ratio but I'll guess it's 9.5:1 judging by your current numbers. This means you should see around 260 psi if I'm correct.

sorry for being of topic

Compression Ratio is measure by taking the volume of 1 chamber at BDC, then taking the volume at TDC and dividing.

Volume @ BDC / Volume @ TDC = Compression ratio

IE

Piston is at BDC < bottom dead center > the measured volume is 10 cubic units. When piston is at TDC < top dead center > the measured volume is 1 cubic unit. This yeilds 10/1 = 10 therefor 10:1 compression. Your engine is taking an area 10 times larger and forcing it into an area of 1.

Compression < measures how good of shape an engine is in. It tells you if you rings are sealing properly, maybe whether there is a hole in a piston etc. When engines wear, rings become less effective, pistons wear and become oval and loose, head gaskets might be leaking. A compression test is usually used to check the life in an engine and how hard it has been run. You can also diagonse simple problems with a compression test. If you do regular compression tests and find your compression dropping, you could try to add some oil into the chamber and retest. If your compression goes up, then it is a sign your rings are worn and maybe need to be replaced.

You can not measure the compression ratio with a compression tester, nor can you measure the compression by a known compression ratio.

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89' Swift GTi Road Race car - 130hp NA - Helical Torsion Diff, 4.71 final Gti guts

12:1 w/o any head work is quite a bit. I was going to do a 12:1 compression total, after pistons and head.

Murr, he won't need 104 octane, although he will need some fine tuning on the cam gears to not ping, probably retard a bit on them.

Talk to Geozuki about high Comp pistons, he is running 12.5:1 total compression in his, overbored and loves the power it produces. He gets no pinging on 94 octane.

Can 12:1 pistons even fit without head work?? i thought they'd smack into the valves! i could be wrong though.

I had suzuki sport 11.5:1 compression pistons and from my understanding these were the highest compression i was able to fit without head work!

Were's our piston specialist on the board!

i don't see why they WOULDN't fit because the head work would actually be SHAVING the head bringing them closer to the top right? lol jus trying to think of simple basics.

There definately will need to be block boring done though in the sleeves thats for sure, they won't jus drop in

I hope he realizes that haha.

I'm curious how you came to this conclusion. I based my statement on personal experience.

My talon comes with 8.5:1 compression stock and the compression reading should be 185 psi.

I've swicthed to 9.5:1 pistons and get higher compression readings now.

The math works perfectly for the readings I now get.

9.5-8.5=1
1/8.5=0.1176
185 X 0.1176=21
185+21=206

I get 205 psi everytime I do a compression test and there is no explanation for the increase besides the change in compression ratio resulting from the new pistons.

If you think of the logic, you're squeezing the same amount of air into a smaller air space so it is going to be under more pressure.

sorry for being off topic

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89' Swift GTi Road Race car - 130hp NA - Helical Torsion Diff, 4.71 final Gti guts

sorry for being off topic

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89' Swift GTi Road Race car - 130hp NA - Helical Torsion Diff, 4.71 final Gti guts

Comparing static compression of different motors is even more ludicrous. Static compression ratio, and dynamic compression are two, totally different things.

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Contact 3tech: [email protected]

Please go back to grade 10 math? How about if you wait until you get to grade 10 math before you started tellin' people to go back. An 11.76% increase in compression ratio increased my compression readings by almost exactly 11.76%.

Check the math. I've got 20 years of school under my belt and I'm an accountant. I do nothing but math everyday. I've taken math classes much more advanced than what you will see as you make your way through high school.

If you're going to say I'm wrong, you could at least back it up with some facts or references instead of telling me to go back to high school.

I used real world experience and have actually had this discussion with some knowledgable people on another bulletin board. What do you have?






Never did I compare between engines, but I did use another engine as an example. It's a 4-cylinder and operates on the same principles. It just happens to be turbo and pushing over 300 horses to all four wheels.

Obviously static and dynamic compression ratios are different things and do you know which one we are discussing here?


You guys should back up your thoughts with some facts and this could turn into a really good thread. Oh and don't start hating on me just because I think a little differently than you. I am totally open to learn more and if I am wrong I'd love to know so I don't go around giving out false information. I was originally just trying to help someone.

Don't worry Mike, we all know they are two, totally different things. The point is that they relate to heach other just like your kids relate to you and your wife.

What this guy is saying is not bullshit. Of course you have the leak percentage to take into acount but still it's minimal on a good condition engine.

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sorry for being off topic

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89' Swift GTi Road Race car - 130hp NA - Helical Torsion Diff, 4.71 final Gti guts

I've got this question for our mathematician - atmospheric pressure (at sea level) is accepted as 15 psi - if I take a given volume of air and squeeze it into 1/10 the volume (static CR of 10:1), the pressue should go up by a factor of 10, so we're looking at 150 psi, right?

That means, that mathematically, there's no way your 8.5:1 Talon could generate 185 psi

Thank you very much. Someone agrees with me (here), I am not alone.







That is excellent. It's too bad that no one has questioned how compression ratio is calculated. I believe the question is how an increase in compression ratio affects compression readings. You said there is no effect and I asked you to back it up. Please enlighten me.

With all your experience you would think you would have had some time around high compression engines. I guess you just never bothered to do a compression test on them. It's too bad because I think if you had, you just might agree with me.

Is there any chance you currently have access to a high compression engine? It would be awesome if you could do a compression test on a high compression engine and compare the readings to the same stock compression engine. And then of course post the results here.

I just happen to have the day off so I can go back and forth with you all day if you choose. With all your posts I assume you'll be around all day too. I will not shut-up until you prove to me that increased compression ratios do not effect compression readings. The reason is because I believe you are spreading misinformation. Every bulletin board I visit is strictly for technical reasons and I would love to learn something here.






I'm not a mathematician. I'm an accountant. I was trying to say my math skills exceed the grade 10 level.

Atmospheric pressure is actually 14.7. I like to calculate pressure ratios and stare at compressor maps in my spare time. Unfortunatley I do not know how to use atmospheric pressure and compression ratio to calculate compression readings. If you do know how then I would greatly appreciate if you enlightened me.

Fords are my first love. Suzukis are a close second though. It is only within the past few years I've been getting into turbo 4-cylinders. The talon just happened to be the one I chose to mess around with.

I would especially love it if a Ford guy could explain the concept abovve to me. My guess is that there must be some constant value that is factored into the formula. Let me know if you know the answer.

Thanks for the great discussion guys. I'm sure people are learning some important concepts from this thread.

i think it will be hard to calculate your new PSI mathmaticly. is 260 going to be close? maybe. but there are other factors involved to say a math formula with tell you exactly what it WILL be.
how many miles on the motor when the compression test was performed? was it warmed up? oil in the cylinders? battery full on charge? pull all the plugs and hold the throttle open?
also on the new motor the type of rings will have an effect on compression. zero gap, high or low tension. and if you are building an all new engine i am sure you will be putting a new cam in. what profile of camshaft you choose will have a huge effect on what cranking compression will be.

either way i cant wait to see a right up of how the build up and finally the test drive goes!!!

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Are you sure about that? A camshaft will affect dynamic compression but has no effect on static compression. Both valves are closed at top dead center and that is what produces the compression reading on the gauge.

I agree with the rings comment though. I hadn't considered that.

positive

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The only way I can see cams affecting the readings is if you advance or retard the cam timing. My logic tells me that under normal circumstances with stock cam timing that a change in the camshaft(s) should not make any difference in compression readings. Don't you agree?

static compression is your compression ratio,thrugh simple math.

http://www.rbracing-rsr.com/compstaticcalc.html

dynamic compression is basicly what you will read on a guage.

http://www.rbracing-rsr.com/comprAdvHD.htm

" Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio."

so, your kinda right,but kinda wrong

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Static compression is a mathematical calculation.

As soon as you get into the real world, you find the inlet valve is typically open with the piston at BDC on the compression stroke, and closes as the piston rises - because of this, some of the volume in the cylinder escapes through the open valve instead of being compressed and this is the primary cause of the difference between dynamic (or actual) compression and static (theoretical/ideal) compression.

Another factor that can change your dynamic compression ratio is the addition of a turbo. This is why I am pretty sure you cannot measure dynamic compression unless your engine is operating. So your regular compression test is in fact not a measure of your dynamic compression ratio.

When there is a turbo the dynamic compression ratio of an engine will change as the level of boost rises and more air is stuffed into those cylinders. This also implies that the dynamic compression ratio will change with RPMs. The efficiency of a particular engine will also effect dynamic compresison. In short I don't think you or I will ever have the resources available to measure dynamic compression ratio. That is if it is even possible.

The compression readings we see are an indication of the engine's static compression ratio and therefore they will change with changes in static compression. This is just as I implied in my original post. The one that started this mess.

you seem to have some real mental block on this!!! oh well your good with numbers on paper. i recomend getting out into the garage and actually putting some of these to use.
racinghart only wanted to know what his cranking compression would be with a fresh engine build. you should check the stickys on the proper way to do a compression check and then PM me to tell me how you will have boost while checking compression with no sparkplugs at about 300 rpm and so on. i am done here.

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Ok People the car is finally finish the compression was 290psi cylinder 1 288 psi cylinser 2 284 cylinder 3 and 288 cylinder 4. The car feel superb up stairs and down stairs. By the way had no pinging problems on 91% octane gas.

Congratulations on getting it together and thank you for reporting back with the results.

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